There are 3 types of engine oils...
Mineral - (Normal)
Semi Synthetic - (Better)
Synthetic - (Best)
Mineral oils should be replaced at sharp 3000Kms, Semi synthetic can last 5000Kms, Fully Synthetic can last up to 8,000kms (Or even more in some expensive brands)
Synthetics are modern oils, they are thin oils, they retain their viscosity very well but they are all just available in v.thin grade such as 0w-30, 0w-40, 5w-40 at max 10w-40
Semi synthetics are usually available in 5w-40 to 10w-40 grade
Any oil above 10w-40 is a mineral oil. 20w-40 & 20w-50 are examples of mineral oils, and these are v.thick oils.
Once you have started using a thick engine oil of grade 20w-40 or 20w-50 you should not try to shift to a synthetic oil and also avoid using semi synthetic oil, specially in older cars. You should continue using 20w-40/50 with a drain interval of 3000Kms sharp.
Going back to lower viscosities and thinner oils can make ur engine seals and gaskits leak. You car can start reducing engine oil as the engine components, seals and washers have now adjusted to a v.thick oil. Shifting back to a thin oil can now casue leaking.
For brand new cars use fully synthetic oil from day one, and change at 5000kms to keep engine happy
For low mielage used cars you can use semi-synthetic in 10w-40
For used cars with higher mileage and unknown oil history, use 20w-40 or 20w-50 of a good brand.
@miansaab I persoanlly dislike all local oils... use a mineral oil by Liqui Moly, Meguin or look for Micking if you want a cost effective oil. You should use 15w-40 or 20w-40/50 in your Mehran.
@zeus65 Personally i thick you should drain 5w-30, as its summers, you car is old and you belong to Sindh were weather is hot. You should not use any oil thinner than 10w-40, may it be semi synthetic or mineral.
Or continue using 20w-50 (shift to a good imported oil brand)
5w-30 is not suitable for this weather, your car and its range is very low 5w-30 is a winter oil (Does not support a wide range of variation in temperature)